The Arctic is melting: Now scientists think they know why (Image: Noaa)

By Alex Kirby
BBC News Online environment correspondent

Scientists believe they have identified a mechanism which can explain the thinning of the Arctic sea ice.

They say the thinning, which in summer reaches more than 40% in some areas, has two causes.

Rising air temperatures, possibly the consequence of global warming, are melting the ice from above. And warmer water is also rising from the depths to attack the ice from below.

Professor Peter Wadhams, of the Scott Polar Research Institute in Cambridge, UK, said in 2000 that he had established the degree of thinning using measurements from submarines in 1976 and 1996.

He said these showed that in that time a large area of the sea ice, stretching from the North Pole to the Fram Strait between Svalbard and Greenland, had thinned by 43% during the Arctic summer.

US data from the other side of the Arctic, between the Pole and the Bering Strait, found a similar thinning over the same period.

Vulnerable in summer

The reported melting has been questioned by some scientists who believe the ice is still there, concentrated in areas where the submarines have not looked for it.

But Professor Wadhams says the thinning he has detected, from 16ft (4.8m) 20 years ago to 9ft (2.7m) today, is scientifically explicable.

Old landmarks will vanish (Image: Noaa)

He told BBC News Online: "People say global warming can't be raising air temperatures enough to melt the ice, because the Arctic winter temperature is around -30C anyway, and a one-degree warming would be irrelevant.

"But it's the summer temperatures that matter. Arctic summers are getting longer, so there is longer for the warmer air to melt the snow and affect the ice beneath.

"The other mechanism is the warming of one or two degrees in the water under the ice - enough to increase the bottom melting quite considerably.

"There is a cold water layer immediately beneath the ice. But that's changing its stability and salinity, because of changes in the distribution of Siberian river water in the Arctic.

Shippers' bonanza

"Over a large area that cold water is becoming more saline and denser, which means it's letting more heat rise through it."

Professor Wadhams thinks the Arctic could be virtually ice-free during the summer by about 2080.

He told BBC News Online: "The north-east passage across the top of Siberia is already close to becoming commercially viable.

Bears are at risk

"It will shorten the existing route from Europe to the Far East by about 40%, from 20,000 km (11,000 nautical miles) to 13,000 km (7,000 nautical miles).

"Containers going from Germany to Japan on Russian vessels are now using that route experimentally. There'll be huge savings for shipping.

"And as the route lies through Russia's territorial waters, it will collect fees for providing ice breakers, search and rescue services and so on."

Threat from the south

But for wildlife the prospects are less good. Polar bears are likely to face problems as the sea ice retreats and makes it harder for them to hunt the seals on which they depend.

Scientists from Norway have begun a long-term programme to tag and monitor bears which, they say, are under threat from both climate change and pollution. They fear man-made chemicals are entering the animals' food chain.

One of the research team, Andrew Derocher of the Norwegian Polar Institute, told the BBC the Arctic was being increasingly polluted by industrial chemicals carried northwards by currents and winds.

Because the chemicals bond well with fat, high levels build up in the seals' blubber. Initial studies show the bears' fertility is being affected by the chemicals.